An airplane cannot take off from a runway which is moving backwards (like a treadmill) at a speed equal to its normal ground speed during takeoff.

busted

First some small-scale tests were performed with a model airplane on a treadmill and the plane was able to take off. For the large-scale test, the MythBusters used a 400 pound ultralight aircraft with a 2000 foot tarp under it. The tarp was pulled backwards to simulate a moving runway. The ultralight pilot had no trouble taking off. This is because the thrust of the airplane engines acts on the air, not on the ground.

A cockroach can survive a nuclear blast.

busted

The MythBusters irradiated cockroaches and several other bugs. The cockroaches all eventually died, and they died sooner than some of the other bugs. Nonetheless, cockroaches would survive longer than humans.

If you freeze a can of shaving cream, cut it open, and then put the foam in a car, it will heat and expand to fill the car.

busted

This myth was first tested with one can of shaving cream; there was little effect. Even with 50 cans of opened shaving cream in the car, the foam did not significantly expand. Lastly, the MythBusters tried putting a large quantity of polyurethane foam in the car, and it expanded to completely fill the car.